How does a domestic micro-CHP boiler work?

Micro combined heat and power (Micro-CHP) technologies generate both heat and electricity at the same time. They have a small fuel cell or heat engine which drives a generator and provides electric power and heat to a building.

At a district scale, marine marine diesel engines modified to run on natural gas are already providing heat and electricity to blocks of flats and retail units, with a recent example in Milton Keynes.

On the micro scale CHP has only recently started to roll out in the UK following numerous false starts, early reliability issues and high unit and installation costs.

Introducing micro-CHP

Most micro-CHP systems operating today run on mains gas or LPG because it’s cheap, it burns easily and it’s accessible. Although natural gas is a fossil fuel, the technology is still considered to be low carbon due to the fact that it can be more energy efficient than simply burning fossil fuel. They are mainly used in homes or small buildings as their electricity generating capacity is relatively small.

The micro-CHP system itself is similar in size and shape to domestic boilers and can be mounted on a wall or on the floor. It can be up to 92% efficient and this is possible because otherwise lost exhaust heat energy is converted into useful electricity by the boiler.

Normally energy escapes as heat when electricity is transmitted through cables and the further electricity has to go, the greater the energy losses. In fact up to to 7%-12% can be lost between a power station and individuals’ homes. Hence, micro-CHP systems can be extremely efficient in the respect that the electricity they generate can be used on-site.

On average a domestic micro-CHP unit will produce 1KW of electricity, with the maximum for a domestic property being 50KW and if you generate more than you can use, any extra electricity can be sold back to the grid.

Micro-CHP technologies

There are two domestic Micro-CHP boiler technologies available today:

(i) The stirling engine is one of the most popular available to the domestic market such as the Baxi Ecogen. It works as follows; when heat is demanded a magnetic piston drives up and down within a generator coil to produce 1kW of electricity to power appliances.

(ii) The fuel cell is to perhaps the most exciting and futuristic. It works by taking energy from fuel at a chemical level rather than burning it and because of this it can offer better energy efficiency and higher rates of electricity generation to heating rates. Although the technology is still at early stage, Viessmann have launched a consumer unit in Germany.

Advantages of micro-CHP

So why would you install a micro-CHP in your home rather than a conventional boiler? Well there are some advantages of micro-CHP

When the micro-CHP is generating heat, the unit is also generating electricity which can be used in your home or exported to the grid.

According to some manufacturers you could save up to £600 on your annual fuel bill.

Installation is easy. In fact, there is very little difference between a micro-CHP installation and a standard boiler. If you already have a conventional boiler then a micro-CHP unit should be able to replace it directly as they’re roughly the same size. All installers must be approved under the micro-generation scheme.

Servicing costs and maintenance are estimated to be similar to a standard boiler, although you will need a specialist installer.

Installing a micro- CHP system in your home also benefits the environment due to the reduction in CO₂ emissions compared to grid electricity and standard boilers. Nearly 73% of CO₂ emissions in the home come from heating and hot water.

Micro-CHP is eligible for FITs so you will earn money for each kWh of electricity your system generates and if you export excess electricity to the grid you will receive an export tariff.

A future role for micro-CHP

With electricity prices rising, it makes economic sense to consider installing a micro-CHP in your home, as it will not only efficiently heat your home, but it will also provide hot water and some useful low cost electricity. In addition, CHP can help with security of energy supply: the electricity generated by micro-CHP units, together with smart grids and energy storage solutions, could help the national grid cope with the peak load requirements. And having a secure energy supply is vital to everyone.

Comments

I looked into the Flow Energy offer and I couldn't make the sums add up. Putting to one side the technology risk for new adopters, the Flow Energy Freedom Package offer is £5445 for a boiler with basic installation. For the first 5 years, Flow keep the FITs but will reduce the monthly energy bill by £80 per month.

Presently, we use about 10000kWhs of gas and 3000kWhs of electricity. We have a 24kW system boiler; smart/zoned heating controls and we live in a 5 bedroomed house. The Flow Boiler is 14kW.

To achieve a zero energy bill for 5 years (total rebate £4800), I would have to increase my gas usage to 30000kWhs. My electricity kWh grid usage would fall to about 1250kWhs.

In otherwords, I would be paying an additional £300 per year for a very hot home and 1750kWhs of electricity (assuming 1750kWhs utilisation which is probably off the mark - but probably more than PV % usage).

Alternatively, I could spend about £2000 less on a top-of-the-range system boiler with a 10 year warranty and use less gas than is the case today.

Given the above, it is difficult to see how a micro CHP boiler would work for the average domestic household. It might make more sense when we get time of use tariffs or for those living in a small terraced house with a swimming pool in the garden.

But would installing one of these and claiming the FIT on it affect the FIT that I am already reciving from my PV panels? I know I am unable to extend them without dropping to the current rate, and would not want to do that with anything else either?